Our long-term objectives are to study mechanisms by which modifier genes affect pulmonary disease severity in patients with cystic fibrosis (CF), with emphasis on the surfactant proteins (SP-), which are produced by lung epithelial cells. SP-A and SP-D play a role in the innate host defense and/or the regulation of inflammatory processes in the lung, and SP-B is essential for normal lung function. Preliminary findings indicate that SP-A genetic variants are associated with disease severity in CF. The overall rationale for the proposed studies is that significant heterogeneity exists in the severity of pulmonary disease in CF patients, even in those that are homozygous for the F508 mutation of the CF transmembrane conductance regulator gene (CFTR). This phenotypic diversity may be related to the genetic heterogeneity of the surfactant protein genes. Genetic variation in the SP-B gene may relate to differences in pulmonary function and genetic variation in SP-A and SP-D may relate to differences in innate host defense function in CF. The overall hypothesis states that SP-A, SP-B, and SP-D are modifiers of pulmonary disease severity in CF and that differences exist among SP-A genetic variants in their ability to modulate phagocytosis of Pseudomonas aeruginosa. In this proposal, we will carry out two major groups of experiments. First, we will study family-based associations by carrying out extended transmission disequilibrium test (ETDT) and/or TDT analyses, of the SP-A, SP-B, and SP-D marker loci, and CF, to determine whether these loci are linked to CF and identify susceptibility SP alleles for CF severity. The entire CF study group as well as a number of subgroups based on CFTR genotype, severity, or other criteria, will be studied. Logistic regression analysis will be used to identify factors that may be of particular significance in CF along with specific genotypes (Aim 1). In the second group of experiments, we will focus on SP-A genetic variants for which preliminary evidence of an association between SP-A alleles and CF severity exists. We will study differences in the ability of SP-A alleles to enhance phagocytosis, by a macrophage-like cell line, of laboratory strains of mucoid and non-mucoid P. aeruginosa grown under different environmental conditions that have been shown to reproduce certain biochemical and functional characteristics found in clinical isolates of P. aeruginosa (Aim 2) and of mucoid and non-mucoid isolates of P. aeruginosa from CF patients (Aim 3). The findings may help identify specific host defense mechanisms involving alleles associated with pulmonary disease severity in CF and develop a useful in vitro model to study the in vivo modifications of P. aeruginosa and its clearance, and provide the basis for further consideration of novel therapeutic strategies to treat CF-related lung disease.